Numerous industrial processes use heterogeneous catalysis reactions. One of the major problems that arises in these processes is that of catalyst deactivation. There are various ways in which heterogeneous catalysts become deactivated, these including the deposition of organic or inorganic compounds, the poisoning of the active sites, and the loss of metals.
Of these deactivation modes, the formation of a deposit on the surface of the catalysts limits the access by the reagents to the active sites and thus gradually reduces the performance of the reactors. This loss of activity is very often associated with the chemical nature of the catalyst, which encourages secondary reactions or polymerization reactions at its surface. In other instances, the precursors of these deposits are formed upstream of the catalytic bed or are impurities present in the reagents, and these become deposited at the top of the catalytic bed.
The generic term “coke” is used to define the formation of a deposit the nature of which is not exclusively organic: this may be carbon-containing deposits resulting from the degradation of the reagents or from impurities present in the reagents, but may also include inorganic compounds such as catalyst dust, including the dust resulting from processes upstream of the catalytic bed concerned. The coke deposit may also contain particles of metals resulting from the attrition or corrosion of the reactors and/or the units sited upstream of the catalytic beds. In most cases, it is organic/inorganic mixtures combining the various modes of catalyst deactivation.
The formation of coke causes a deterioration in the performance of the catalyst and therefore limits the yield in terms of reaction product.
The loss of activity of the catalyst entails periodically shutting the plants down in order to regenerate the catalyst, for example thermally, and this leads to a lack of productivity of the industrial plant.
When the coke has its precursors upstream of the catalytic bed, one industrial solution for limiting the deactivation of the catalyst is to use a sacrificial bed upstream of the catalytic bed. A sacrificial bed such as this may consist either of catalyst or of beads of chemically inert solid. This sacrificial bed fixes the coke deposits before they reach the catalytic bed. The sacrificial bed is then periodically replaced so as to maintain catalyst performance. In some cases, several successive layers may be stacked up to optimize reactor operation. This is the case, for example, in the hydrotreatment of petroleum cuts; layers of inert solids are used to fix the largest particles, notably of metals produced by the corrosion of the reactors, or the gums produced by the polymerization of unsaturated molecules. Layers of active solids are used to trap metals such as vanadium, nickel, arsenic or sodium which are known poisons of hydrotreatment catalysts. These various layers are placed above the catalysts involved in the hydrotreatment, and thus limit the deactivation thereof.
There are other solutions that have been proposed in order to avoid the deactivation of catalysts through the formation of coke. Patent application EP 1 714 955 describes a method for eliminating the deposition of solid organic substances in a fixed-bed multi-tube reactor, which method consists in introducing a solid that has a Hammett acidity of between −5.6 and 1.5 at the top of the catalyst-filled tube, or between the layers of catalysts, or as a mixture with the catalyst; this method is suited to gas phase catalytic oxidation reactions, more particularly to the oxidation of propylene to acrolein in the presence of molecular oxygen, followed by the oxidation of acrolein to acrylic acid.
In patent application EP 1 734 028, a process for producing acrolein and acrylic acid from propylene or propane using an oxidation reaction in the presence of a catalyst is performed using filtered air as the source of oxygen. Filters, for example of the metal gauze type, are used upstream of the process in order to eliminate any particles present in suspension in the air needed for the process.
Other solutions, like the one described in U.S. Pat. No. 6,545,178, consist in limiting the formation of by-products that are the root cause of the deposits on the catalyst, by using raw materials that are free of impurities. This method does, however, entail prior purification of the raw material, such as propylene for producing acrolein/acrylic acid, and this leads to a process oncost.
In general, conventional industrial processes use, upstream of the catalytic bed, a bed of small-sized beads made of a ceramic material, which are not necessarily chemically inert, as a solution to the problem of catalyst deactivation through the deposit of coke. The beads are simply laid on top of the catalytic bed in each tube of the multi-tube reactor. They are replaced by, for example, sucking out the bed of beads, and tremendous precautions have to be taken in order not to damage the catalytic bed lying under the bed of inert beads. This operation is not only expensive in terms of time, but also exposes personnel. In addition, using suction to unload beads which are often firmly bound together with coke, destroys the beads which have therefore to be replaced each time.
Surprisingly, it has been found that it is possible to limit the deactivation of catalysts during catalytic reactions in multi-tube reactors by using particle filters of monolithic structure known for their application to diesel particle filters.
A subject of the present invention is therefore the use of monoliths such as particle filters to limit the deactivation of catalysts during catalytic reactions in multi-tube reactors, said monoliths comprising parallel channels the walls of which are made of porous ceramic, and the inlet cross section of which is greater than or equal to the outlet cross section.
The monoliths, preferably made as a single piece, are simply placed on the catalytic bed. They can therefore easily be removed and replaced. It is far easier to replace a monolith made as a single piece than, for example, to have to suck up a bed of beads taking extreme care not to damage the catalytic bed that lies under the bed of inert beads.
The monoliths also lead to a lower pressure drop than beds of beads. What happens is that the pressure drop is generated only by the thickness of the wall of the monoliths, which is far thinner than the bed of beads equivalent to the length of the monolith.
Other features and advantages of the invention will become better apparent from reading the description which follows and from referring to the attached figures in which: